removed the restriction about draw buffer 0 having to be non-NULL if
any higher draw buffers are non-NULL. Appears this can be supported
with a NULL FOURCC render target. Removed issue and adjusted
introductory text.
Review URL: https://codereview.appspot.com/6725048

git-svn-id: https://angleproject.googlecode.com/svn/trunk@1292 736b8ea6-26fd-11df-bfd4-992fa37f6226